1. Field of the Invention
The present invention relates to data processing systems and, in particular, to for detecting the existence of repetitive data sequences in a received signal using pattern recognition. The invention has particular application for detecting training sequences for Quadrature Amplitude Modulation (QAM) modem receivers.
2. Discussion of the Prior Art
The basic function of any communications system is to transmit information over a communication channel from an information source to a destination as fast and as accurately as possible.
There are two general types of information sources. Analog sources, such as a telephone microphone, generate a continuous signal. Digital sources, such as a digital data processing system, generate a signal that consists of a sequence of pulses.
Communications channels that are designed to transmit analog signals (e.g., the telephone network) have characteristics which make it difficult for them to transmit digital signals. To permit the transmission of digital pulse streams over an analog channel, it is necessary to utilize the digital data pulses to modulate a carrier waveform that is compatible with the analog transmission channel.
The equipment that performs the required modulation is generally referred to as a "MODEM". The term "MODEM" is an acronym for MOdulator-DEModulator, since one piece of equipment typically includes the capability not only to modulate transmitted signals, but also to demodulate received signals to recover the digital data from the modulated analog carrier waveform.
While passing through the transmission channel, the modulated carrier waveform suffers from distortion introduced both by the system itself and by noise contamination. Thus, one of the tasks of the modem's demodulating receiver function is to filter the signal received from the transmission channel to improve the signal-to-noise (S/N) ratio. The modem receiver also recovers timing information from the received signal to provide sampling points for recovering the digital data. The modem receiver may also condition the data in other ways to make it suitable for additional processing.
In a conventional modem, the signal filtering, timing recovery and conditioning tasks are performed by three functional units: analog-to-digital conversion circuitry ("analog front end") that converts the received modulated carrier waveform to a digitized replica, a digital signal processor (DSP) that retrieves the digital data from the digitized replica using a recovered timing signal, and a control function for controlling both the analog front end and the DSP. The DSP recovers the data by implementing a signal conditioning and data recovery algorithm that is specific to the type of data being received.
Typically, the modem's filtering function is "adaptive". That is, the data symbols of the received signal are used to continuously update the coefficients of the filter's transform function to improve its performance. The initial setting of these coefficients, for example, at system power-up, is usually accomplished in an iterative "training" procedure. This procedure involves transmitting a training sequence to the modem to allow the modem receiver to adjust its filter coefficients to the desired values. The training sequence is preceded by a repetitive signal that alternates between two levels to indicate the existence of the training sequence.
A major problem in "training" a modem receiver's adaptive filter is the presence of noise in the observed output values. Thus, the early detection of the existence of a training sequence in a noisy received signal with a specified Signal to Noise threshold (S/N) is very important for modem implementation.
As stated above, a conventional modem receiver uses DSP or an analog implementation of band pass filters and adaptive detection to detect energy above the specified S/N threshold.
The DSP implementations require many time-consuming multiplication operations.
Analog implementations are difficult to implement in today's high density integrated circuits.